iwlwifi/fw: use struct_size over open coded arithmetic

Replace zero-length array with flexible-array member and make use
of the struct_size() helper in kzalloc().

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi (CGEL ZTE) <chi.minghao@zte.com.cn>
Link: https://lore.kernel.org/r/20220216030841.1839666-1-chi.minghao@zte.com.cn


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>